This invention pertains generally to soldering fluxes and methods, and is more particularly concerned with a water soluble soldering flux and a method using the same, which offer significant environmental benefits as well as significant production-related advantages, especially in the manufacture of critical, high reliability electronic assemblies such as printed wiring assemblies.
Printed wiring assemblies are commonly produced in a process known as wave soldering, in which surfaces of a printed wiring board (printed circuit board) and of component leads and terminations to be soldered thereto are subjected to a solder wave which passes along the board and components mounted thereon. The solder wave contacts the surfaces to be soldered, and bonds and electrically connects them together. Prior to the actual soldering of such assemblies, a flux is ordinarily applied to the surfaces to be joined in order to chemically prepare the surfaces to receive the solder, as by removing oxides, so that a good electrical and mechanical connection will be established. Typically, the flux is applied in somewhat loosely controlled amounts by so-called foam fluxing, in which the board and mounted components are passed along a foaming bed of flux which deposits on the surfaces to be soldered. The fluxed assembly is pre-heated prior to soldering to volatilize the flux carrier from the surfaces.
Soldering fluxes used by the electronics industry in the wave soldering of printed wiring assemblies have traditionally contained ingredients which are detrimental to the environment and/or which necessitate the use of post-soldering cleaning agents which themselves present undesirable environmental risk. For example, typical activated rosin fluxes utilize volatile organic compound (VOC) carriers, such as alcohols, which pollute the environment. Such fluxes may also leave substantial post-soldering contaminant residues of an ionic nature (traces of flux and its decomposition and reaction products) which can eventually corrode and otherwise physically and electrically degrade electronic assemblies, resulting in circuit failure. Thus, particularly in the case of critical, high reliability assemblies, the removal of such residues has been essential. Cleaning has perhaps most commonly been accomplished by vapor degreasing using chlorofluorocarbon (CFC) cleaning agents. The environmental hazards of CFC's, and particularly their recently understood harmful effects on the Earth's stratospheric ozone layer, have become a major worldwide concern.
As the environmental consequences of electronics production processes have become better understood, and in order to meet increasingly strict government regulations, the art has sought to avoid the above-described problems by developing alternative cleaning agents as well as by developing improved flux formulations which may be cleaned with safer, environmentally friendly cleaning agents, such as water.
For example, various water soluble fluxes have been proposed which may be cleaned by washing with water. While water cleaning is obviously preferable to CFC cleaning as a general proposition, prior water soluble fluxes have generally left much to be desired. For example, such fluxes have typically been formulated with substantial amounts of VOC solvents (commonly alcohols) and/or halide-containing activators. As noted earlier, VOC solvents are problematic from an environmental point of view. Halide-containing activators, on the other hand, are problematic in that they leave highly ionic, highly corrosive post-soldering residues which must be cleaned. More particularly, in the presence of humidity (water), the halides contained in such residues are known to act somewhat as catalysts in a regenerative corrosion reaction in which the halides complex with the metal (e.g., aluminum) of assembly components and are then regenerated by reaction of the halogenated complex with water, thereby becoming free to attack the metal again. Thus, so long as the humidity remains present (and humidity is, of course, almost always present in practice), the corrosion reaction may proceed unremittingly, ultimately resulting in circuit failure.